During the 1170s. The Normans moved into Ireland and steadily pushed back the native Irish. Their military superiority was based on the quality of their armoured and mounted fighting men, and on the part played in their campaigns by castles.
As developed by the Normans, the castle was a multi-purpose instrument of war and government. It provided a stronghold in which a lord and his men could sleep safely in a hostile land, and a place of refuge if their enemies gained the upper hand. But a castle might also function as a forward post in general advance, securing fresh territory, deterring local resistance, and serving as a base for inspecting or surveying the enemy and a further advance against the enemy. Finally, when the countryside was pacified, it was a highly visible symbol of dominance, leaving no one in doubt about who was master.
The Normans believed so strongly in the value of castles that they put up temporary structures as quickly as possible. (They had done the same in England after 1066.) Unskilled local labour could be used to throw up a ditch-encircled mound of earth and stone (the motte), on which a timber tower and palisade were raised; if this was linked with a secondary, ground-level enclosure (the bailey), the result was the well-known motte-and-bailey type of castle.
Many of the mottes can still be identified, but the timber towers vanished long ago. By about 1200 some lords were already building in stone, raising great rectangular keeps (towers) with first-floor living quarters where the lord and his family could try to sit out a siege; the keep was the place of last resort, the front line of defence being the strong wall around it, all the more effective if it was reinforced at intervals by wall-towers from which crossfire could be directed against the enemy. The castle of Carrickfergus (County Antrim) was built on this pattern in the early 1200s, while the biggest of all Norman-Irish castles, at Trim (County Meath), was completed a few decades later, in time to incorporate more up-to-date ideas in the form of wall-towers with round outer faces, less vulnerable than corners to undermining activities. A circular keep was built, for the same reason and probably at about the same time, at the spectacularly sited Dundrum (County Down).
After this, castle-construction followed the prevailing English style, with powerful gatehouses, portcullisees (strong grating, as of iron, made to be let down to prevent passage), barbicans (a fortification to a castle outside the walls, at end of the drawbridge in front of the gate), machicolations (projecting gallery or parapet as on a wall through which missiles, molten lead might be cast upon an enemy beneath). Royal castles were most technically advanced, dispensing with keeps and relying on mighty walls with strong round towers (each of which was in effect a keep) to guard and overawe towns such as Dublin, Limerick and Roscommon. The early advandage held by the invaders waned as the native Irish learned how to build in Anglo-Norman fashion - quite directly in the case of Ballintober, erected by the O'Conors of Connaught at the end of the 13th century, after they had twice captured, occupied, studied and eventually destoyed nearby Roscommon Castle.
(Splendidly preserved, Carrickfergus Castle stands on a rocky spur above Belfast Lough, It has a large keep and two surrounding curtain walls, Begun late in the 12th bentury, it was captured by King John in 1210 and served intermittently for centuries as a royal castle. Carrickfergus last saw action in 1760, when it was briefly occupied by French raiders).
(Trim Castle is the largest Anglo-Norman fortess in Ireland).